The invention relates to thyristors, and, more particularly, to thyristors having both turn-on and turn-off capabilities.
Thyristors are well known semiconductor devices which are typically used as current switches for enabling and interrupting current flow in a circuit. A thyristor is "turned-on" when it provides a high conductance path between two of its terminals (that is, is anode and cathode), and is "turned-off" when it provides a high resistance path between such two terminals. A typical prior art thyristor 10 is shown in FIG. 1. The thyristor 10 incldues four regions, 12, 14, 16 and 18, of alternating conductivity type; an anode 20 and a cathode 22; and a metal-oxide-semiconductor ("MOS") turn-on structure 24; or, more broadly stated, a conductor-insulator-semiconductor turn-on structure 24.
The MOS turn-on structure 24 includes a gate 26 and an insulating layer 28 separating the gate 26 from the semiconductor body of the thyristor 10. The biasing of the gate 26 with a positive voltage (with respect to the cathode 22) exceeding a threshold value "inverts" the portions of the P-type region 16 adjacent to the insulating layer 28, thereby creating inversion channels 30, which can conduct electrons. Accordingly, electrons from the cathode 22 can flow in the electron current paths 32 through the N-type region 18 to the N-type region 14, via the inversion channels 30.
As is known in the art, the thyristor 10 can be modeled as two thyristor structures, comprising an N-P-N transistor structure, formed by N-type region 14, P-type region 16, and N-type region 18, and a P-N-P transistor structure, formed by P-type region 12, N-type region 14, and P-type region 16. The N-P-N and P-N-P transistor structures are regeneratively "coupled" to each other; that is the collector of the N-P-N transistor structure (region 14) is coupled to the base of the P-N-P transistor structure (region 14) and so can drive this base, and the collector of the P-N-P transistor structure (region 16) is coupled to the base of the N-P-N transistor structure (region 16) and so can drive this base. Accordingly, the supply of electrons to the N-type region 14 (the base of the P-N-P transistor structure) causes both the N-P-N and P-N-P transistor structures to regeneratively turn on, whereby the thyristor 10 becomes turned on.
The above-referenced patent application discloses and claims a semiconductor device which operates as a thyristor and which includes MOS turn-off structure at its lower surface (corresponding to the lower surface of the thyristor 10).
It would be desirable to provide a semiconductor device which can operate as a thyristor and which has MOS turn-on structure at its upper surface and MOS turn-off structure at its lower surface. Such a device would have turn-off capabilities and yet be simple to manufacture because each surface of the device would have only two electrodes; that is, a gate for MOS turn-on or turn-off structure and the anode or cathode.